1. Field of the Invention
The present invention relates to a method and apparatus for optical scanning, and more particularly to a method and apparatus for optical scanning capable of reducing a dot position displacement and a dot diameter variation based on a variation in wavelength of a laser beam.
2. Discussion of the Background
Background optical scanning apparatuses for use in various image forming apparatuses including laser printers, facsimile machines, and digital copiers have a drawback in which a focal image position on an imaging surface is varied and consequently an image quality on the imaging surface or on an output recording sheet is degraded. This variation in focal image position on the imaging surface is caused when a wavelength of a laser beam generated by a laser light source is varied and a refractive index of optical devices are accordingly changed.
A technique described in Japanese Laid-Open patent publication, No. 09-021944 attempts to avoid the effects from the variations in wavelength using a dispersion nature of a glass lens to improve an accuracy of output images.
On the other hand, an optical scanning apparatus capable of simultaneous multi-beam scanning using a plurality of laser light sources has been developed in response to an increasing trend for a high speed image reproduction. In the multi-beam method, an image quality may be degraded when the multiple beams are not precisely synchronized which is caused in many cases by variations in wavelength of the laser beams.
A technique described in Japanese Lain-Open patent publication, No. 09-076562, relates to an optical scanning lens to attempt to avoid the above-mentioned problem by reducing a difference in wavelength between multiple beams below 12 nm.
Also, in recent years, a demand for a high image density in the image reproduction using an optical scanning apparatus is rapidly increased and, to implement this demand, requirements for reduction of a dot size and a dot position displacement have grown.
There is a further problem in that an image quality is degraded due to variations in wavelength of a laser beam caused by changed in temperature. That is, when the wavelength of a laser beam is changed, an optical scan-imaging device such as fxcex8 (ef-theta) lens changes its refractive index and, as a consequence, a start position in the scanning line on an imaging surface is changed which causes a displacement of dot position.
The above-mentioned Japanese Lain-Open patent publication, No. 09-021944 proposes a glass-made scanning lens which prevents a deterioration of an image forming accuracy by a reduction of chromatic aberration of the scan-imaging device. However, it is difficult to achieve a similar effect using a plastic lens, which falls into a complex lens structure.
In view of the foregoing, it is an object of the present invention to provide a novel optical scanning apparatus which reduces a displacement of dot position and variations in dot diameter to output a superior quality image.
Another object of the present invention is to provide a novel optical scanning method which reduces a displacement of dot position and variations in dot diameter to output a superior quality image.
To achieve these and other objects, in one example, the present invention provides a novel optical scanning apparatus including a pulse modulation mechanism, a laser light source, a light deflecting mechanism, and a scan-imaging device. The pulse modulation mechanism is configured to perform a pulse modulation with respect to input image data to output pulse-modulated image data. The laser light source is configured to generate a laser light beam in accordance with the pulse-modulated image data output by the pulse modulation mechanism. The laser light beam has a wavelength variation smaller than 2.0 nm per one pulse of the laser light beam. The light deflecting mechanism is configured to deflect the laser light beam generated by the laser light source to convert the laser light beam into a scanning laser light beam. The scan-imaging device is configured to condense the scanning laser light beam into a scanning light spot on a surface to be scanned.
Another novel optical scanning apparatus to achieve the above-mentioned object, in one example, includes a pulse modulation mechanism, a laser light source, a light detecting mechanism, and a scan-imaging device. The pulse modulation mechanism is configured to perform a pulse modulation with respect to input image data to output pulse-modulated image data. The laser light source is configured to generate a laser light beam in accordance with the pulse-modulated image data output by the pulse modulation mechanism. The laser light beam has a wavelength variation greater than 0.5 nm per one pulse of the laser light beam. The light deflecting mechanism is configured to deflect the laser light beam generated by the laser light source to convert the laser light beam into a scanning laser light beam. The scan-imaging device is configured to condensing the scanning laser light beam into a scanning light spot on a surface to be scanned. In this structure, the pulse-modulated image data applied to the laser light source from the pulse modulation mechanism is controlled so that a displacement of dot position and variation in dot diameter caused by the wavelength variations are corrected. The present invention also provides novel optical scanning methods.